BAGHDAD — Thirteen people were killed and 45 were wounded when a small parked motorcycle detonated early Friday in a crowd of young men in the Bab al Sheikh neighborhood in downtown Baghdad, Iraqi police said.

The explosion, which occurred only four days before the scheduled withdrawal of U.S. troops from Iraqi cities is to be completed, raised more concerns about the readiness of Iraqi forces to control security, especially as it came amid a wave of violence across the country.

Gen. Abdulkarim Khalaf, an Interior Ministry spokesman, accused unnamed parties of attempting to put obstacles in the way of the American withdrawal.

"They started mobilizing their criminal organizations to carry out attacks in order to obstruct the U.S. forces' withdrawal, but this will not happen. The withdrawal will continue as it was planned," he said.

The explosion at around 9 a.m. on a quiet Friday targeted a crowd of mostly young men who'd gathered to buy and sell motorcycles.

Ibraheem Nomas, 51, an employee of a garage close to the explosion site, saw shrapnel flying into the garage and ran to help the injured.

"When I reached the place of the explosion, I saw six dead men. Their bodies were completely burnt by the explosion. I saw a young man crying out for help for his young brother, who was still burning. I helped in saving four young men," he said.

"Ambulances arrived quickly and saved many people, but many were already dead," he added. "Whoever did this is not human."

Nomas isn't optimistic about the future of Iraq. He said he thought that the security situation was going to worsen in the coming days.

"The withdrawal of the American forces will leave a security gap that the terrorists can use to attack Iraqis."

Prime Minister Nouri al Maliki said at a news conference Wednesday that terrorists would "try to kill the happiness in the hearts of Iraqis," but he attempted to assure the public that the country would be safe.

"The Iraqi security agencies can and have been handling the security situation for some time now," he said, adding, "The role of the American forces has long diminished."

President Barack Obama, speaking in Washington, agreed that security largely had improved in Iraq.

"If you look at the overall trend, despite some of these high-profile bombings, Iraq's security situation has continued to dramatically improve," he said.

However, Ali Hussein, who witnessed Friday's bombing, had his doubts.

"The withdrawal of the U.S. forces is a very bad idea because the security situation is not stable at all, and our security forces are not ready yet," he said.

(Hammoudi and Issa are McClatchy special correspondents. Margaret Talev contributed to this article from Washington.)